adc-24 differential measurement, 4-20mA

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adc-24 differential measurement, 4-20mA

Post by mikael.petersson »

Good morning.

Need some advice.

I’m using a ADC-24 to log a 4-20mA signal from a light meter.

To do that I’m measuring the voltage over a 125 ohm resistor (actually 2 parallel 250 ohm 0,005%) connected in parallel to the meter. I’m using channel 1 and 2 in a differential config set at 2,5volts.

My problem is (that I’m not sure what I’m doing) the voltage I get is to low. During nights when its dark where the current from the meter should be close to (not under) 0.5volt (=4mA) I get readings from 470 to 498mV (about 3,75 to 3,98mA) and jumping from second to second, not keeping very stable.
The meter is calibrated and according to calibration documentation from manufacturer 4mA is 0lux. And as I understand it the picolog adc-24 is factory calibrated.

So any ideas where to start looking. Of cause the meter (and picolog) could be faulty but I would like to rule out other possible problems before. Is my thinking wrong not to use the ag connection.

The adc-24 terminal board manual shows an example (page 4: 2.1.1) differential voltage divider measurement where the AG is used, but I’m not “voltage dividing”. Should I be using two 62,5 ohm resistors instead?? One over input 1 to AG and one over AG to input 2 and the connect logger over input 1 and 2???

Hope somebody have any ideas?

Thank you

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